1. Field of the Invention
The present invention relates to an antenna device and a radio apparatus operable in multiple frequency bands, and in particular to a built-in type antenna device and a radio apparatus including the antenna device.
2. Description of the Related Art
There is a trend that mobile phones or personal computers (PCs) having radio capability have multiple purposes and multiple functions. The above trend requires an antenna device which may be operable in multiple frequency bands or in a broad frequency range.
In order to meet the above requirement, antenna devices designed to have multiple resonant frequencies (to be operable in multiple frequency bands) or to be operable in a broad frequency range are disclosed, e.g., in Japanese Patent Publication of Unexamined Applications (Kokai), No. 2004-172912 or No. 2004-201278.
More specifically, JP 2004-172912 discloses a multi-frequency (multi-band) antenna of an inverted F type formed by a feeding line, a short-circuiting line and a first open-ended line. The antenna of JP 2004-172912 further has a second open-ended line almost shaped into a rectangle and arranged on an opposite side of the feeding line as viewed from the first open-ended line. According to JP 2004-172912, it has been estimated by simulation that the antenna configured as described above may have resonances, e.g., at a 2.4 gigahertz (GHz) band and at a 5.2 GHz band.
JP 2004-201278 discloses a pattern antenna including an inverted F type antenna, an inverted L type antenna and a ground conductor which are conductive patterns formed on a surface of a printed board. The inverted F type antenna may be fed and excited. The inverted L type antenna is arranged to nearly surround the inverted F type antenna and may be excited as a parasitic element. According to JP 2004-201278, resonant frequencies of the inverted F type antenna and the inverted L type antenna may be determined by their element lengths so that the pattern antenna may have at least two resonant frequencies.
JP 2004-172912 discloses an embodiment of the multi-band antenna applied to a wireless local area network (WLAN). The arrangement of the second open-ended line being nearly rectangular and the first open-ended line on the one side and on the other side of the feeding line, respectively, may cause a parallel resonance. If the multi-band antenna is used in a lower frequency band such as a mobile phone antenna, the parallel resonance may disturb a broadband characteristic there.
As described above, the parasitic element of the pattern antenna of JP 2004-201278 is arranged to nearly surround the inverted F antenna of an element length being shorter than the length of the parasitic element. It may thus be understood, according to a paragraph “0035” of JP 2004-201278, that the inverted F antenna is arranged close to the parasitic element along a whole element length of the inverted F antenna.
If an element to be fed and a parasitic element are arranged in positions relative to each other as described above, though, it may be difficult to excite the parasitic element at a desired frequency due to effects of a voltage-coupling and a current-coupling which may cancel each other. If open ends of both of the elements are arranged separate in order to avoid such difficulty, it may be difficult to shape a radio apparatus including the elements as a built-in antenna into a low profile configuration.